Since 1960, contamination of an operating room by anesthetic gas and adverse effect of the anesthetic gas on the health of workers in the operating room have been taken as a matter of issue. It is known that as a result of long term inhalation of the anesthetic gas leaked in the operating room, the health is disordered. The anesthetic gas means a mixed gas containing nitrous oxide, a volatile anesthetic and oxygen, and the waste anesthetic gas means anesthetic gas after the respiration of a patient. The composition of the waste anesthetic gas is approximated to the composition of the anesthetic gas and contains a volatile anesthetic, nitrous oxide of high concentration, and oxygen. In the U.S.A., the National Institute of Occupational Safety and Health (NIOSH) recommends to reduce, as a permissible standard, nitrous oxide (N2O) to 25 ppm or less and a volatile anesthetic to 2 ppm in the case of a sole use and to 0.5 ppm or less in the case of a combination use with nitrous oxide. Accordingly, all anesthesia machines are obliged to be equipped with a waste anesthetic gas removing apparatus and at the present time, the environment in the operating room can reach the above-described levels.
The waste anesthetic gas removing apparatus is an apparatus for discharging the waste anesthetic gas outdoors from the exhalation of a patient by letting a compression air or the like to accompany the gas. However, the gas discharged from each operating room by the waste anesthetic gas removing apparatus is released into atmosphere without passing through any treatment at the present time.
The waste anesthetic gas discharged from an operating room differs from the nitrous oxide-containing exhaust gas discharged from factories or incineration facilities in the following two points:
(1) the concentration of the nitrous oxide contained in the waste anesthetic gas is very high and from 3 to 70%, and
(2) the waste anesthetic gas contains a volatile anesthetic gas.
Among volatile anesthetics, volatile anesthetics containing chlorine are said to destroy the ozone layer. Also, in the third session of the Conference of the parties (COP3), nitrous oxide is, as well as nitrogen dioxide, methane and chlorofluorocarbon, particularly taken notice of as a global pollutant which brings about elevation of temperature due to greenhouse effect (the warming effect is as high as about 300 times the carbon dioxide).
Accordingly, from the standpoint of global environment protection, the waste anesthetic gas should not be released into atmosphere as it is using the waste anesthetic gas removing apparatus but both volatile anesthetic and nitrous oxide contained in the waste anesthetic gas must be removed or rendered harmless.
With respect to conventional methods for treating the waste anesthetic gas, (1) a method of decomposing the nitrous oxide contained in the waste anesthetic gas is described in JP-B-61-45486 (the term “JP-B” as used herein means an “examined Japanese patent publication”), JP-B-61-45487, U.S. Pat. No. 4,259,303 (JP-B-61-50650 and JP-B-62-27844). Also, (2) a method of cooling and thereby removing volatile anesthetics contained in the waste anesthetic gas is proposed by Arai et al (see, Rinsho Masui (Clinical Anesthesia), Vol. 1, No. 1, page 98 (1977), Fujita Gakuen Igaku-kai Shi (Journal of Medical Society of Fujita Educational Institute), Vol. 5, page 117 (1981)). Furthermore, (3) a method of thermally decomposing the nitrous oxide by a Nichrome wire heating is reported as a method of treating nitrous oxide contained in the waste anesthetic gas without using a catalyst (see, Masui (Anesthesia), No. 28, page 1242 (1979)).
According to the method (1) of decomposing nitrous oxide, nitrous oxide in a high concentration may be decomposed but nitrogen monoxide (NO) and nitrogen dioxide (NO2) (hereinafter collectively referred to as “NOx”) as nitrogen oxides are produced in an amount of 5 to 32 ppm, thus, generation of NOx in excess of the allowable concentration of 3 ppm (TWA, time weighted average) for NO2 remains as a problem. In addition, when moisture in an amount of, for example, approximately from 1 to 3% is present in the reaction gas, the catalyst may decrease in the activity, and this remains as a problem to be solved.
When a waste anesthetic gas is fed as it is to a nitrous oxide decomposition catalyst, decrease in the specific surface area of the nitrous oxide decomposition catalyst is incurred in some cases to cause extreme reduction of the activity. The reasons therefor is not clearly known but it is presumed that acids generated at the decomposition of the volatile anesthetic deactivate the nitrous oxide decomposition catalyst. Accordingly, in order to maintain the activity of the nitrous oxide decomposition catalyst, volatile anesthetic must be removed, however, a method for effectively removing volatile anesthetic contained in the waste anesthetic gas is heretofore not known.
The above method (2) of cooling and thereby removing volatile anesthetic contained in the waste anesthetic gas proposed by Arai et al has a problem in the profitability because a volatile anesthetic gas removing device and a cooling unit for cooling the removing device must be installed in each operating room. In addition, the installation of a large apparatus such as cryogenic refrigeration unit in an operating room is not preferred from the location and hygienic aspects. Furthermore, in the case of treating a large amount gas flown from the pipeline to which the waste anesthetic gases discharged from respective operating rooms are converged, there arises a problem that the volatile anesthetic cannot be satisfactorily removed. From these reasons, this method is not used in practice.
The above method (3) of thermally decomposing nitrous oxide by the Nichrome wire heating without using a catalyst is not preferred for the use in hospitals in view of safety, because the reaction temperature is as high as 900° C., a neutralization cleaning unit is necessary, and the concentration of NOx generated is very high and around 0.1%.
A process and an apparatus capable of continuously treating volatile anesthetic and nitrous oxide contained in a relatively large amount in the waste anesthetic gas discharged from each operating room have heretofore not been known. To keep up with the current increase in concern for the global warming due to nitrous oxide, development of a process and an apparatus capable of continuously treating a waste anesthetic gas containing volatile anesthetic gas and nitrous oxide is being demanded.
The present invention made under these circumstances is to provide a process and an apparatus for treating a waste anesthetic gas containing a volatile anesthetic and nitrous oxide discharged from an operating room.